This invention relates to a method and apparatus for control of a load commutated inverter-synchronous machine drive system, and more specifically, to a method and apparatus for control of a synchronous machine drive system without utilizing a shaft position sensor, as more fully described in our corresponding paper "Load Commutated Inverter Synchronous Motor Drive Without a Shaft Position Sensor" published in the Conference Record of the IEEE-IAS Conference in Los Angeles, Calif. in October 1977.
Inverter synchronous machine drive systems, to which the present invention is directed, are typically comprised of a synchronous machine, which may be of the inductor type, excited by variable frequency alternating voltage produced by an inverter from a source of direct current potential. The inverter used in such systems is typically comprised of a plurality of pairs of serially connected controlled unidirectional conduction means, such as a thyristor, corresponding in number to the number of machine phases. Each pair of thyristors is coupled in parallel across a DC source and is connected at the junction therebetween to a corresponding machine phase for controlling current conduction therein. When gated in a preselected sequence, the thyristors will conduct in a manner to provide alternating current of varying phase for machine excitation.
Inverter synchronous machine drive systems, such as described above, are presently enjoying renewed popularity in certain applications such as flywheel energy storage for electric vehicles. An important reason for such renewed popularity is that such machine drive systems may be made self-commutating, that is to say, that inverter thyristors may be extinguished by back electromotive force generated by the synchronous machine during operation, thus eliminating the need for large external commutating components.
To insure full machine performance under load, it is often important to synchronize the inverter-machine drive system, and, to adjust inverter thyristor gating intervals to vary the frequency of machine excitation in response to machine load variation.
In the past, various approaches have been disclosed to provide machine-inverter synchronization. In U.S. Pat. No. 3,796,935 issued Mar. 12, 1974 to Felix Blaschke and assigned to Siemens Aktiengesellschaft of Germany, machine inverter synchronization is provided by a pair of Hall generator probes for sensing machine shaft position coupled to appropriate associated feedback circuitry for generating a feedback angle command signal to vary inverter thyristor firing responsive to machine shaft position. A major disadvantage present within the apparatus of Blaschke is that machine shaft position, representing the actual machine phase angle, is sensed by Hall generator probes and thus requires machine modification for accommodation therein.
Still another approach for stabilizing an inverter-synchronous machine drive system is disclosed in U.S. Pat. No. 4,088,934, issued May 9, 1978 to J. D. D'Atre et al., and assigned to the assignee of the present invention. D'Atre et al. achieve machine-inverter synchronization by first deriving torque and angle signals representing the actual machine torque and phase angle, respectively. The actual torque and phase angle signals are compared to a torque command and subsequently-generated phase angle command signal, and a resulting feedback error signal, representing the magnitude of the difference therebetween, is impressed on the inverter for adjusting inverter thyristor gating intervals accordingly. Although the necessity for Hall probe or shaft position sensors, as taught by Blaschke, is eliminated, special function generators are required to generate appropriate torque and phase angle command signals for adjusting inverter firing. Additionally, D'Atre et al. do not employ apparatus for synchronizing an inverter-synchronous machine subject to operator-commanded load variation. The present invention does not require a shaft position sensor or special function generators to achieve synchronization, and provides inverter-synchronous machine synchronization in response to operator-commanded load variation.